[Tails-dev] Clarify the link to the signature of the Torrent…

As pointed out on the forum [1], the way the download page was linking
the cryptographic signature of the Torrent file was confusing for some
users. I tried to fix that with commits e4d69fd.

Can you please review the changes? They are already visible on the
download page [2].

[1] https://tails.boum.org/forum/Clarifying_a_silly_but_confusing_detail/

[2] https://tails.boum.org/download#download_iso